1. Field of the Invention
The present invention relates to a rechargeable lithium ion battery. In particular, the present invention relates to an electrode assembly that prevents the separator from contracting due to heat that is generated near the electrode tab during overcharging or over-discharging, or in the case of an internal short circuit. The present invention also prevents additional short circuits from occurring between the electrode plates. The invention also relates to a rechargeable battery that uses the electrode assembly.
2. Description of the Prior Art
A rechargeable battery refers to a battery that can be repeatedly charged and discharged, in contrast to a non-rechargeable battery that cannot be recharged. Rechargeable batteries are widely used in cutting-edge electronic devices including portable telephones, laptop computers, and camcorders.
For example, lithium ion rechargeable batteries have an operating voltage of 3.7 V, which is three times larger than that of nickel-cadmium batteries or nickel-hydrogen batteries which are often used in portable electronic devices. The lithium ion rechargeable batteries also have a high energy density per unit weight which makes them suitable for use in the industry.
Lithium ion rechargeable batteries usually use a lithium-based oxide as the positive electrode active material and a carbon material as the negative electrode active material. Lithium ion rechargeable batteries are generally classified according to the type of electrolyte used such as liquid electrolyte and polymer electrolyte. Batteries that use a liquid electrolyte are referred to as lithium ion batteries and batteries that use a polymer electrolyte are referred to as lithium polymer batteries. Lithium ion rechargeable batteries are manufactured in various shapes including cylinder-type, can-type, and pouch-type batteries.
A can-type lithium ion rechargeable battery, as shown in FIG. 1 and FIG. 2 includes a can 10, an electrode assembly 20 contained in the can 10, and a cap assembly 70 that seals the opening of the can 10.
The can 10 may be made of metal and may be in the shape of a rectangular box. Thus, the can may itself serve as a terminal of the battery. The can 10 has an opening that is formed on one of its surfaces such as a top opening 10a, through which the electrode assembly 20 is placed into the can 10.
The electrode assembly 20 includes a positive electrode plate 30, a negative electrode plate 40, and a separator 50. The positive electrode plate 30 and negative electrode plate 40 are laminated with the separator 50 that is interposed in between them and are wound into a jelly roll.
The positive electrode plate 30 includes a positive electrode collector 32 that is made up of thin aluminum foil and a positive electrode coated portion 34 that has a lithium-based oxide as its main component which is coated on both surfaces of the positive electrode collector 32. The positive electrode collector 32 also has a positive electrode uncoated portion 32a that has no positive electrode coating formed thereon and is positioned on both ends of the positive electrode plate 30. A positive electrode tab 36 is fixed to the positive electrode uncoated portion 32a by ultrasonic welding with both ends thereof being fixed to protrude from the upper end of the positive electrode collector 32. The positive electrode tab 36 usually comprises nickel or nickel alloy, but other metallic substances may also be used.
The negative electrode plate 40 includes a negative electrode collector 42 that is made of thin copper foil and a negative electrode coated portion 44 that mainly comprises a carbon material and which is formed on both surfaces of the negative electrode collector 42. The negative electrode collector 42 has a negative electrode uncoated portion 42a that has no negative electrode coating formed thereon and is positioned on both ends of the negative electrode plate 40. A negative electrode tab 46 is fixed to the negative electrode uncoated portion 42a by ultrasonic welding with both of its ends protruding from the upper end of the negative electrode collector 42. The negative electrode tab 46 usually comprises nickel or nickel alloy, but other metallic substances may also be used.
The separator 50 is positioned between the positive electrode plate 30 and negative electrode plate 40 to insulate them from each other. The separator 50 may comprise polyethylene, polypropylene, or a composite film of polyethylene and polypropylene, for example. The separator 50 may be wider than the positive electrode plate 30 and negative electrode plate 40 to prevent a short circuit between them.
The cap assembly 70 includes a cap plate 71, an insulation plate 72, a terminal plate 73, and a negative electrode terminal 74. After being coupled to a separate insulation case 79, the cap assembly 70 is coupled to the top opening 10a of the can 10 and seals it.
If a rechargeable battery is over-charged, over-discharged, or if a short circuit occurs between the electrodes, heat may be generated in the can. In particular, heat generation is concentrated on a part of the can that experiences increased internal resistance. This is the region where different kinds of metals are bonded to each other to weld the electrode plate to the electrode tab. Since heat generation is concentrated near the electrode tab, the separator that insulates the positive electrode plate and the negative electrode plate from each other may melt and contract. As shown by a dotted line 52 in FIG. 2, the end of the separator 50 that is positioned on the negative electrode tab 46 may severely contract and the insulation film between the positive electrode plate 30 and negative electrode plate 40 may disappear. As a result, an additional short circuit may occur between the positive electrode plate 30 and the negative electrode plate 40. When heat is generated in the rechargeable battery, the end of the separator 50 that is positioned on the positive electrode tab 36 may also contract.
As recently developed rechargeable batteries tend to have a larger power storage capacity, the energy density of these batteries also increases. This results in a higher occurrence is of over-heating and explosion of the rechargeable batteries.